Automobile body panels are made by shaping low carbon steel or aluminum alloy sheet stock into inner and outer panel shapes. The number of sheet metal pieces that must be formed and welded or otherwise attached together to form the vehicle body depends upon the design shape of the panels and the formability of the sheet metal. It is desirable, both from the viewpoint of manufacturing cost and fit and integrity of the assembled structural panels, to make the body from as few parts as possible. Other manufacturing operations are likewise affected by the complexity of a product shape that can be formed from the starting sheet metal. Thus, there is always an incentive to devise more formable metal alloys and better forming processes so that relatively few parts of more complex shape can be made and joined to make a car body or other product rather than welding or bolting together a myriad of smaller, simpler pieces.
R. L. Hecht and K. Kannan made an assessment of using superplastic forming (SPF) of a commercial SP aluminum alloy 5083. This work and assessment is described in their publication, "Mechanical Properties of SP 5083 Aluminum After Superplastic Forming" in the monograph, Superplasticity and Superplastic Forming, published by The Minerals, Metals and Materials Society in 1995. They used an AA5083 that had been processed to exhibit superplasticity and they observed that the alloy exhibited high elongation when tested uniaxially at temperatures of 500.degree. C. and above at strain rates of 10.sup.-4 sect.sup.-1 to 10.sup.-3 sec.sup.-1.
Hecht and Kannan formed front cross member reinforcement brackets for automobiles by superplastic forming. The SP 5083 brackets were formed at 490.degree. C. with 0.45 MPa (65 psi) gas pressure on a male forming tool without back pressure. They reported a forming time per part of approximately 40 minutes. While their practice formed a part of complex shape in a single step, the time required was far too long for practical automobile manufacturing applications.
Later, Nakamura et al of Honda R&D Co. and related Honda companies reported the superplastic hot-blow forming of a boat hull using an aluminum alloy of AA5083-like composition. Their work was published as "A new process for small boat production based on aluminum hot-blow forming (ABF)", Journal of Materials Processing Technology, 68 (1997) 196-205. The AA5083-type alloy (aluminum with 4.5% magnesium and small amounts of manganese and chromium, and the impurities iron and silicon) exhibited high elongation at temperatures between 510.degree. C. and 550.degree. C. and strain rates of 10.sup.-4 sec.sup.-1 to 10.sup.-3 sec.sup.-1. The Honda workers required half an hour to one hour to complete forming of the boat hull. Again, the SPF process as used permitted the forming of a complex shape but the strain rate was too low and the cycle time too long for automobile manufacturing.
The U.S. Pat. No. 4,645,543 to Watanabe et al. describes a process for making modified AA5083 sheet material having "excellent superplasticity." These alloys were composed, by weight, of 3.5% to 6% magnesium; 0.12% to 2% copper; at least one of 0.1% to 1% manganese, 0.05% to 0.35% chromium, and/or 0.03% to 0.25% zirconium; and the balance of aluminum and unavoidable impurities. Maximum incidental amounts of many other elements are also specified. After chill casting and a carefully specified schedule of hot rolling followed by cold rolling, some 18 different superplastic sheet samples, 1.6 mm thick, were made for testing.
The Watanabe et al. superplastic aluminum-magnesium-copper alloy samples were prepared as tensile test bars, heated to 530.degree. C. and subjected to an initial strain rate of 1.1.times.10.sup.-3 /sec to determine total superplastic elongation. Among the many alloy samples, total elongation values of from 330% to 800% were obtained.
The low strain rate of the Watanabe et al. superplastic tensile test specimens is typical of superplastic forming strain rates for these magnesium-containing aluminum alloys as reported in the Hecht et al and Nakamura et al publications. Just as the Watanabe tests would take many, many minutes in order to determine the final elongation at 530.degree. C., SPF forming operations on modified AA5083 sheet metal stock have taken 30, 40 or 60 minutes or more to form into a shaped article.
It is an object of this invention to provide a high strain rate, stretch forming process for high elongation (superplastic), magnesium-containing aluminum alloys, like AA5083, to enable the practical production of robust automobile body panels and the like of complex shape and highly strained regions. While this practice was devised for automobile manufacture, it can obviously be used to make other usable articles.